NOx Reactions on Aqueous Surfaces with Gaseous HCl: Formation of

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NOx Reactions on Aqueous Surfaces with Gaseous HCl: Formation of a Potential Precursor to Atmospheric Cl Atoms Audrey Dell Hammerich,*,† Barbara J. Finlayson-Pitts,‡ and R. Benny Gerber‡,§,∥ †

Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, United States Department of Chemistry, University of California Irvine, Irvine, California 92697, United States § Institute of Chemistry and the Fritz Haber Research Center, The Hebrew University, Jerusalem 91904 Israel ∥ Laboratory of Physical Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Helsinki, Finland ‡

S Supporting Information *

ABSTRACT: Chlorine atoms are highly reactive free radicals known to catalyze ozone depletion in the stratosphere and organic oxidation in the troposphere. They are readily produced photolytically upon irradiation of some stable Cl containing species, for instance, nitrosyl chloride, ClNO. We predict the formation of ClNO using ab initio molecular dynamics (AIMD) simulations of an NO2 dimer on the surface of a thin film of water upon which gaseous HCl impinges. The reactant is chloride ion formed when HCl ionizes on the water film. The same mechanism for ClNO production may occur in humid environments when ONONO2 (the asymmetric NO2 dimer examined here) comes in contact with either HCl or sea salt. The film of water serves to (1) stabilize ONONO2 on the film surface so that it is localized and physically accessible for reaction, (2) provide the medium to ionize HCl, and (3) activate ONONO2 making it more susceptible to nucleophilic attack by chloride. This substitution/elimination mechanism is new for NOx chemistry on thin water films and could not be derived from studies on small clusters. SECTION: Environmental and Atmospheric Chemistry, Aerosol Processes, Geochemistry, and Astrochemistry

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route to generate a chlorine atom precursor warrants further elucidation, in particular the role of water. The only theoretical model proposed for ClNO formation is based upon studies of small clusters containing a few water molecules at most. As such, the role of an aqueous film remains elusive. This work gives an atomistic model of a thin water film upon which a molecule of ONONO2 is adsorbed. Employing density functional theory (DFT), its reaction, or lack thereof, is followed dynamically as gaseous HCl collides with the surface. The more realistic water model allows the role of the aqueous surface to be deduced and the mechanism of ClNO formation to be captured. Results on model compounds are compared with higher level ab initio calculations to assess the veracity of the current DFT calculation. In this study, ClNO is shown to be produced by a three-step mechanism: (1) a thin water film activates the ONONO2 molecule adsorbed on its surface, making it more amenable to nucleophilic attack, (2) gaseous HCl collides with the aqueous surface and promptly ionizes, and (3) the liberated chloride ion attacks the nitroso nitrogen atom of the asymmetric dimer and substitutes with simultaneous elimination of nitrate. Water assumes a preeminent role in this mechanism. It enables ionization of

ormation of species such as ClNO in air is potentially very important since photolysis in sunlight generates chlorine atoms. Atomic chlorine is even more reactive than the OH radical and accelerates the complex organic-oxides of nitrogen cycles that lead to the formation of ozone and other secondary pollutants.1,2 Chloride ion is known to react with N2O4 to produce nitrosyl chloride and nitrate.3 It was also suggested that ClNO could form in the atmosphere when moist NaCl comes into contact with nitrogen dioxide.4 Experiments conducted with relatively high levels of NO2 (1.5%) reacting with wet or dry NaCl indeed evidenced ClNO as a product.5 More germane to atmospheric conditions was the demonstration that nitrosyl chloride formed when parts per million (ppm) levels of NO2 reacted with dry NaCl.6 In both sets of experiments, the formation of ClNO plateaued, suggesting a surface controlled reaction. Recent experiments show that HCl reacts with surface associated NO2 in a water mediated reaction that also forms ClNO.7 The reaction and its surface specificity is consistent with a mechanism postulated for the heterogeneous hydrolysis of NO2. First, symmetric nitrogen dioxide dimer, N2O4, forms on a surface; in the presence of a thin film of water it isomerizes to its asymmetric form, ONONO2. This subsequently autoionizes to the reactive nitrosonium nitrate ion pair, NO+NO3−,8 which reacts with RCl (e.g., R = H, Na) to generate ClNO. The mechanism leading to this potential new © 2012 American Chemical Society

Received: September 23, 2012 Accepted: November 6, 2012 Published: November 6, 2012 3405

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HCl and formation of Cl−, anchors the ONONO2 so that it is localized and physically accessible for reaction, and, most importantly, activates the dimer for reaction by providing a reservoir for charge transfer that facilitates a redistribution of charge within the dimer with resulting geometrical changes. The bond breaking and bond making necessary to produce ClNO from HCl and ONONO2 cannot be treated within the framework of conventional empirical potentials. Ab initio molecular dynamice (AIMD) is one of two main computational approaches applicable to such systems9−12 along with the empirical valence bond method.13,14 Yet, few AIMD studies have been published on the N2O4 system, and the majority of those that have model a water cluster rather than a film of water. An on-the-fly MP2/DZV investigation of the ionization of ONONO2 in clusters of 0−8 H2O molecules showed dramatic enhancement in the rate of NO+NO3− formation with the number of water molecules in the cluster.15 A dynamic B3LYP/6-31G study indicated that clusters of N2O4 with 0−7 H2O isomerized in water via assisted and nonwater assisted pathways.16 A quantum mechanics/molecular mechanics (QM/ MM) study with ONONO2 and 0 or 2 waters treated at the CASSCF(12e/9o)/6-31G** level of theory with the remaining 279 H2O constituting a classical water box evidenced significant charge separation between the NO and NO3 parts.17 Clusters of ONONO2, HCl, and 0−2 H2O in an MP2/cc-pVDZ molecular dynamics simulation yielded a mechanism where hydrogen-bonded water formed a proton wire for H+ transfer from HCl to H2O to the NO3 moiety forming HNO3 and nitrosyl chloride.7,18 Numerous electronic structure studies have been performed on N2O4 systems17,19−32 yielding information on equilibrium geometries and electronic and thermodynamic parameters. A few studies addressed the effect of water by examining the system in a small water cluster7,15−18 or using an approach trying to incorporate the effect of bulk water.17,25,26 To the best of our knowledge, an AIMD study explicitly incorporating a water surface upon which NOx chemistry can occur has not been reported. In general, the cluster studies do suggest an incipient autoionization of cis-ONONO2. However, as will be shown for the current system of interest, their limited size can overestimate the effect of specific proton transfer events, especially when a strong acid such as HCl is incorporated into the cluster. Without a larger aqueous reservoir capable of participating in charge transfer and supporting the concurrent geometry changes, a mechanism deduced from these smaller clusters may not necessarily extrapolate beyond the reduced dimensionality physical model. The simulations reported here are obtained with the publicly available CP2K suite of programs (http://www.cp2k.org). The electronic structure is calculated with the Quickstep module,33 which implements the hybrid Gaussian plane wave method for ab initio Born−Oppenheimer molecular dynamics within the Kohn−Sham framework of DFT.34,35 The Becke−Lee−Yang− Parr36,37 functional incorporating the Grimme dispersion correction,38,39 BLYP-D3, is used. The Kohn−Sham orbitals are expanded in a triple ζ valence basis with two sets of polarization functions (TZV2P) and core electrons replaced by norm conserving atomic pseudopotentials optimized for BLYP.40 Elements of the Kohn−Sham and overlap matrices less than 10−12 are neglected, a 280 Ry electron density grid is employed, and a convergence criterion of 10−6 is placed upon the electronic gradient. The nuclear equations of motion are integrated with a 0.5 fs time step.

DFT, within the generalized gradient approximation corrected for dispersion, has been shown to yield a good description of water at the BLYP-D3/TZV2P level of theory employed here.41 To assess its performance on the N2O4 system of interest, molecular parameters and partial atomic charges are obtained for the structures given in Figure S1 and summarized in Tables S1 and S2 of the Supporting Information. The bond lengths and angles compare well with higher levels of theory (distances within 0.02 Å, angles within 3°) with one exception. The N1−O3 bond of the cis isomer in Figure 1 is calculated to be 10% higher than the value from a

Figure 1. Left: Optimized gas phase structures and Mulliken partial atomic charges (au). Values in parentheses are from averaging a 20 ps NVE trajectory with the dimer on the aqueous surface. Right: Dynamics of isomer on the thin film surface. (a) Distance (Å) of N atoms in each fragment from center of mass in z direction, blue line denotes Gibbs dividing surface; (b) ON−ONO2 bond (Å) (N2O3 bond of cis-ONONO2 shown on left); (c) angle between planes defined by the two NO2 groups and N−O−N bridge angle; (d) angle from surface normal of plane defined by NO3 O atoms and of NO bond.

CASSCF(12e/9o)/6-31G** optimization (1.47 versus 1.34 Å). This bond is not broken during the reaction, so its greater length should have a minimal impact upon ClNO formation. The current DFT investigation recovers the same stationary structure product as the MP2/cc-pVDZ level theory where ClNO was formed from the reaction of HCl with cis-ONONO2 in the presence of one water molecule.18 The recovered cisONONO2/H2O/HCl structure in Figure S1 is in excellent agreement with the higher level theory structure, even to within 0.01 Å in bond lengths. The largest variation is 3.5% for two hydrogen bonds. The magnitudes of the partial atomic charges reported in Table S2 vary with different theoretical approaches and type of population analysis. It is the change in charge distribution during the course of the simulated reaction that is most useful in examining the reaction mechanism and understanding the electron density that defines the reactive species and sites susceptible to reaction. Here the Mulliken population scheme is used. The AIMD simulations use a 13.4724 × 15.5566 Å2 rectangular box with the z dimension varying between 38 and 40 Å. The system is periodic in xy, the free dimension exceeding twice the atomic layer. This box size enables a convergent treatment of the long-range Coulomb interactions between species42 at the cost of performing part of the calculation over an empty volume. A slab of 72 deuterated 3406

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blue line denotes the 90° angle for alignment parallel to the slab surface. The nitrate plane is tilted so that the reactive nitroso end protrudes into the vapor slightly above zGDS with the NO bond angle generally tracking the plane. In spite of variations, Figure 1c shows how correlated motion in the two ends of ONONO2 is. Omitting the final few picoseconds, the angle between the planes formed by each NO2 group is loosely constrained around 150° (180° would be flat) and the ON− O−NO2 bridge angle is even more constrained about 117°. Though the majority of this activity is normal thermal motion, it is enhanced by the loosely bound nature of the fragments as seen in Figure 1b. The bond length during the first 5 ps is 1.9 and then lengthens to 2.2 Å until almost the end of the trajectory. The location and orientation of ONONO2 on the water surface and the resulting partial charge distribution within the dimer define the nature of its reaction with HCl. In all trajectories, HCl dissociates and forms a hydronium ion within the first 2−8 ps when it is near the Gibbs dividing surface. The reaction, when it occurs, is due to attack by the dissociated Cl− on the relevant, positively charged N atom of ONONO2. While the relative kinetic energy of the reactants is important, a key factor determining reactivity is the direction of attack with respect to the position of the N atom. Several trajectories are considered, showing the dependence on energy and orientation. Limiting behaviors are summarized in the left panels of Figure 2: (a) slow reaction (313 K trajectory with 2

water molecules is equilibrated at 300 K for 40 ps, a geometry optimized molecule of cis-ONONO2 placed on top, and an additional 20 ps equilibration in the canonical ensemble performed. All H in the simulations are deuterated to increase stability, lessen quantum-nuclear effects, and enable utilization of a larger propagation step size. To initiate collision a molecule of HCl is positioned in one of three orientations 4.5−6.5 Å above the Gibbs dividing surface, zGDS. A 2 ps NVT equilibration is done with the z dimension constrained at the initial position above zGDS. Then 1, 2, or 4 kT of collision energy is imparted at the start of a microcanonical simulation. Thirty 10−50 ps NVE trajectories are collected. Dynamical simulations of cis-ONONO2 on the surface of a thin aqueous film show that surface water molecules enable inter- and intramolecular charge transfer so that the dimer can partially ionize. This effect is seen upon comparing the gas phase and aqueous columns of data in Table S1 of the Supporting Information. When placed on water, the NO3 fragment becomes more similar to NO3− and the NO to NO+. This is observed in the 0.03 Å lengthening of the nitrate N1−O1 and N1−O2 bonds, 0.12 Å shortening of the N1−O3 bond, and slight decrease of the nitroso N2−O4. Similarly the O1N1O2 nitrate bond angle closes by 6°, while the other two angles open by 3−4°. Even more dramatic is the 0.4 Å increase in the ON−ONO2 bond between the two fragments (N2−O3 bond). A QM/MM study found an even greater extension of 0.5 Å; however, the isomer was not on the water surface but immersed within.17 The small water cluster studies exhibited similar behavior albeit to a less pronounced extent.15,17 While the bond lengths and angles are not equal, as would be expected for the nitrate ion, they are all converging in the direction of increasing charge separation in the two fragments. Further evidence of charge separation is shown by the Mulliken partial atomic charges (Table S2). In all cases, the N atoms bear a positive charge and O atoms a negative one aside from the almost neutral nitroso O. In the gas phase, the NO3 fragment has a negative charge and the NO fragment a positive one of the same magnitude. On water, the NO3 negative charge doubles in value, and the adsorbed molecule bears a small net partial negative charge from charge delocalization with surrounding waters, seen in Figure 1. This behavior has been reported in higher level calculations with clusters and a QM/ MM study.15,17 The changes in structural parameters and partial charge distributions evidence a profound influence of water. Simulations of ONONO2 on an aqueous surface furthermore show that motion within the isomer and its configuration on the film surface serve to anchor the dimer in an arrangement, which enhances its subsequent chemical reactivity. The right panels of Figure 1 give a 20 ps temporal evolution for various groups of atoms of cis-ONONO2 on a thin aqueous film surface at 306 K. Figure 1a follows individual N atoms of the two fragments. The horizontal blue line denotes the Gibbs dividing surface obtained by fitting the z coordinate data in the density profile for water O atoms to a hyperbolic tangent function.43 From the fit, ρ = 1.01 g/mL, zGDS = 4.7 Å, and the interfacial thickness parameter δ = 0.5. For the unoccupied lower slab surface, zGDS = −5.4 Å, and the slab is 10.1 Å thick. The panel indicates that the isomer resides on the vapor side of the dividing surface and, from crossings of the red and black lines, tilts along its length. Figure 1d defines the orientation. The average angle for the plane of the NO3 O atoms from the normal to the surface is 50°, and for the NO bond it is 68°. The

Figure 2. Left: Three limiting cases monitored by distances (Å) of reacting species. Color coding: N−Cl (black curve), N−N (red), ON−ONO2 (green). (a) Slow reaction, (b) fast reaction, (c) no reaction. Right: (d,e) Mulliken partial atomic charges (au) of Cl and nitroso N for (f) a fast reaction similar to that in panel b. Origin of time is 3 ps into the reaction.

kT collision energy), (b) fast reaction (311 K with 4 kT), and (c) no reaction (298 K with 4 kT). In Figure 2c, clearly the N2−O3 bond stays intact, maintaining its 2.1 Å bond length with no separation between the nitrate and nitroso fragments, additionally indicated by the constant 3 Å N−N distance. Until 4 ps, Figure 2b shows similar behavior (ledge formed in the red and green curves between 1 and 2 ps is due to lengthening of the N2−O3 bond observed in Figure 1). The ensuing N−Cl distance decreases to a 2.1 Å minimum for the N−Cl bond, and the two distances indicative of separation of the nitroso and nitrate fragments increase, hallmarks of a concerted simultaneous substitution/elimination reaction: Cl− approaches to bond with the nitroso N, while the ON−ONO2 bond concurrently lengthens until the NO3− group is expelled. The trajectory in Figure 1a is similar to the previous two until 6 ps. Then the N−Cl and ON−ONO2 distances change until both 3407

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Figure 3. Left Panels: snapshots during fast reaction of initial state (0 ps), [ON(Cl)ONO2]− transition state (4.5 ps), and separated products (11 ps); Right Panels: Wannier centers (black spheres) before and after transition state. Color coding: Cl (green), N (blue), ONONO2 and reaction product O (red), and water H/O (yellow).

(large gold spheres), one sees four WFC’s (black spheres) associated with the O atom where each center corresponds to an electron pair, two for the two bonding pairs to H atoms and two for the two lone pairs. This classical bonding description can be extended to the nitroso N before and after the transition state showing that, in spite of its length, the nitroso N is indeed bonded. Before the transition state, the black oval encircles the bridging ON−ONO2 bond where the Wannier center is localized next to the more electronegative O atom in the N−O bond. After the transition state the WFC is positioned next to the Cl atom of the newly formed Cl−NO bond. In both cases, the bonding electrons are in closer proximity to the more electronegative element in accord with classical theory. The simulations yield a three-step mechanism: (1) ionization of gaseous HCl on an aqueous surface, (2) water activation of ONONO2, and (3) substitution of Cl− in consort with elimination of NO3−. Upon ionization, chloride ion attacks the water activated NO nitrogen. The [ON(Cl)ONO2]− complex exists for tens of femtoseconds to tens of picoseconds eventually separating into ClNO and NO3−. The simulations all exhibit HCl ionization with H+ protonating neither ONONO2 nor the ClNO and NO3− products. In one case, HNO3 transiently forms but reionizes on a femtosecond time scale. Direct protonation of a dimer O atom, rapid Cl− substitution onto the nitroso N, and then rapid dissociation into HNO3 and ClNO (akin to the cluster mechanism) could only be observed for HCl collision energies in excess of 20 kT. For those trajectories that yield no reaction, the chloride ion either goes into the interior of the slab or stays on the surface/subsurface with about equal probability. This is consistent with the slight surface preference (1.6 times) predicted for Cl−.46,47 The methodology employed in the current work is applicable to a broad range of aqueous surface chemistry. Here an important atmospheric reaction is examinedthe product ClNO is a Cl atom precursor produced from HCl and an N2O4 isomerand unveils a mechanism unobtainable from small cluster models. While ClNO has yet to be detected in the atmosphere, those of a related compound, nitryl chloride (ClNO2), which is a product of HCl and N2O5,7 has been.48,49 Hopefully the laboratory and theoretical demonstrations of ClNO formation will spur efforts for its detection and quantization in the atmosphere.

are about 2.4 Å. These distances prevail for the next 16 ps at which time an N−Cl bond forms when the separation falls to 2.1 Å and the N−N and ON−ONO2 distances grow in magnitude. Here the system has more difficulty in releasing the nitrate anion and recurrences in shorter N2−O3 distances are seen in the neighborhood of 24 ps. The right panels of Figure 2 view the rapid reaction through change in partial charges as the reaction unfolds. The trajectory in Figure 2f is similar to the fast reaction in Figure 2b. Figure 2d,e portrays Mulliken partial charges on Cl and the NO nitrogen in a 5 ps interval around the collision event. Figure 2d shows that the initial 0.5 ps is characterized by large fluctuations in Cl charge. During this time, HCl is deprotonating with femtosecond periods of back and forth proton transfer to a neighboring water molecule. Values closer to zero signify when the proton is localized on Cl (covalent Cl) and the more negative values has HCl ionized leaving ionic Cl−. At the minimum, Cl− is 4 Å from the nitroso N (and the proton is nowhere in sight). During this time interval, the charge on the nitroso N is approximately constant. Then charge transfer begins: as the chloride ion forms a covalent bond with the NO nitrogen, there is a transfer of electron density as evidenced by a 0.20−0.25 au increase in the Cl partial charge. Figure 2e shows a decrease in the nitroso N partial charge over the same time period. The charges level off as the ClNO product separates. Figure S2 in the Supporting Information monitors the partial charges on the NO3 and NO fragments. There the extent of charge indicates that NO3− is formed, and the NO progresses from a more positive to less positive environment. Snapshots of the fast reaction in Figure 3 provide a physical qualitative description of the reaction. At 0 ps, ONONO2 is adsorbed on the surface, held by hydrogen bonds to the water molecules. At this stage, the impinging HCl molecule is intact and is not in contact with the surface. Some picoseconds later, HCl reaches the surface and ionizes (not shown). The situation shown at 4.5 ps is considered to be the transition state configuration. Cl− interacts with water but also with the NO+ group, now undergoing separation from the NO3−. Finally, separation of the products is complete in the snapshot at 11 ps; the ClNO formed leaves the water surface, with which it weakly interacts while the NO3− ion is adsorbed and strongly Hbonded at the surface. The exceedingly long ON−ONO2 and Cl−NO bonds encountered in the simulations may be disconcerting. The right two panels of Figure 3 provide substantiation for their existence. The panels give the centers of the maximally localized Wannier functions (WFC)44 before and after the transition state. The black ovals encompass the centers (black spheres), the positions of the maximum probability of finding an electron.45 Examining one of the water oxygen atoms

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ASSOCIATED CONTENT

* Supporting Information S

Structures of model compounds and their partial charges; cis isomer ab initio structural data; partial charges on NO3 and NO fragments for fast reaction. This material is available free of charge via the Internet at http://pubs.acs.org. 3408

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Helpful discussions with Doug Tobias and James N. Pitts Jr. are very gratefully acknowledged. J.N.P. is also thanked for review of the manuscript. This work was performed under NSF Grant 0909227 with computational facilities made possible by NSF Grant CHE-0840513. A.D.H. and R.B.G. acknowledge partial support by Israel Science Foundation Grant No. 114/08.

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dx.doi.org/10.1021/jz3014985 | J. Phys. Chem. Lett. 2012, 3, 3405−3410